In producing optical fibres the aim is to provide the fibre with a coating as soon as possible after drawing. This coating usually being of silicon rubber is used for increasing the fibre strength, so that it can be handled without risk of breakage during reeling and during a following cable manufacture. Immediately after the glass fibre has been drawn from a so-called preform, or from a crucible, and it has had time to cool somewhat, it is passed through a bath of curable silicon rubber. The silicon rubber layer which adheres to the glass fibre during passage through the bath is heat cured, after which the filament can be wound on a reel. As long as the glass fibre passes through the bath at a rate of about 0.4 m/sec this coating method functions well, and the fibre is coated with a uniform layer of silicon rubber.